The invention relates generally to the field of photography, and in particular to dual film image and electronic image capture cameras. More specifically, the invention relates to a dual film image and electronic image capture camera with electronic image verification of film image misfocus.
Recently, a dual image or hybrid camera has been proposed for substantially simultaneously capturing a film image of a subject being photographed and an electronic image of the subject. The camera includes a film image capture or taking lens, an electronic image capture lens, and an image display that shows a captured electronic image of the subject which corresponds to a captured film image of the subject. The displayed electronic image permits one to verify they got the captured film image they wanted. Thus, it is important that the captured electronic image have substantially the same sharpness or degree of focus as the captured film image.
When the film image capture lens is focused on the subject to give a sharp image of the subject, other objects in the scene closer to or farther from the lens than the focused subject do not appear equally sharp. The decline of sharpness is gradual and there is a zone extending in front of and behind the focused subject where the blur is too small to be noticeable and can be accepted as sharp. This zone of acceptable sharpness is known as the depth of field of the lens. The limits of the depth of field of a lens are given by the known equations (in Encyclopedia of Photography, Third Edition, Stroebel and Zakia, (copyright) 1993 by Butterworth-Heinemann, page 197):
Dnear=uf2/(f2+NCu)
Dfar=uf2/(f2xe2x88x92NCu)
where: Dnear is the near limit of the depth of field of the lens;
Dfar is the far limit of the depth of field of the lens;
u is the focused distance;
f is the focal length of the lens;
C is the circle of confusion; and
N is the f-number of the lens aperture. If the depth of field (DOF) is defined as Dfar-Dnear, then the previous equations may be combined and simplified to
DOF=2u2NC/f2
The depth of field is directly proportional to the circle of confusion (C), the f-number (N), and the square of the focused distance (u), and is inversely proportional to the square of the focal length (f).
Typically, the dual image camera includes rangefinder coupled focusing, i.e. automatic focusing, in which a rangefinder first determines the focused distance (u) when a shutter release button is partially depressed, and a focusing mechanism then automatically focuses the film image capture lens at the focused distance when the shutter release button is further depressed. The electronic image capture lens is often separate from the film image capture lens, and has a depth of field (DOF) with near (Dnear) and far (Dfar) limits that are spaced farther apart than the near and far limits of the depth of field of the film image capture lens. Consequently, when the rangefinder misranges to a far object farther from the camera than the subject to automatically focus the film image capture lens on the far object (rather than on the subject), the captured electronic image of the subject can appear to be acceptably sharp or in focus in the image display provided the subject remains in the depth of field of the electronic image capture lens. However, the captured film image of the subject will be blurred or out of focus when the subject is between the near limit of the depth of field of the film image capture lens and the camera. This typically occurs when the subject is positioned off-center in the scene and the rangefinder unintentionally misranges to a centered object in the background of the scene. Alternatively, when the rangefinder misranges to a near object closer to the camera than the subject to automatically focus the film image capture lens on the near object (rather than on the subject), the captured electronic image of the subject can appear to be acceptably sharp or in focus in the display provided the subject remains in the depth of field of the electronic image capture lens. However, the captured film image will be blurred or out of focus when the subject is farther from the camera than the far limit of the depth of field of the film image capture lens. This will occur when the subject is positioned off-center in the scene and the rangefinder unintentionally misranges to a centered object in the foreground of the scene.
Generally speaking, in a dual image camera the invention assures that the captured electronic image shown in an image display will at least substantially match the captured film image in regard to sharpness when the film image capture lens is misfocused because of unintended misranging. Thus, the displayed electronic image will indicate the captured film image is out of focus.
Preferably, the invention assures that the captured electronic image will match the captured film image when the film image capture lens is misfocused on a far object farther from the camera than the subject because of unintended misranging to the far object, and/or when the film image capture lens is misfocused on a near object closer to the camera than the subject because of unintended misranging to the near object
When the film image capture lens is misfocused on a far object farther from the camera than the subject because of misranging to the far object, the electronic image capture lens is focused according to the equation
Ue=Dnearffe2/(fe2xe2x88x92DnearfNeCe)
where: Ue is the focused distance for the electronic image capture lens;
Dnearf is the near limit of the depth of field of the film image capture lens;
fe is the focal length of the electronic image capture lens;
Ce is the circle of confusion for the electronic image capture lens; and
Ne is the f-number of the lens aperture of the electronic image capture lens, when the subject is between the near limit of the depth of field of the film image capture lens and the camera.
When the film image capture lens is misfocused on a near object closer to the camera than the subject because of misranging to the near object, the electronic image capture lens is focused according to the equation
Ue=Dfarffe2/(fe2+DfarfNeCe)
where: Ue is the focused distance for the electronic image capture lens;
Dfarf is the far limit of the depth of field of the film image capture lens;
fe is the focal length of the electronic image capture lens;
Ce is the circle of confusion for the electronic image capture lens; and
Ne is the f-number of the lens aperture of the electronic image capture lens, when the subject is farther from the camera than the far limit of the depth of field of the film image capture lens.